Create PNG image with background consisting of squares in two colors

Question

i'm tring to create PNG image with background consisting of squares in two colors, the squares is repeated horizontally or vertically as in the picture.

This is my code :

<?php
$maxwert = 300;
$size = 20;
$img = imagecreatetruecolor($maxwert, $maxwert);
imagecolorallocate($img, 0, 0, 0);

   for($y=0;$y<$maxwert;$y += $size){
     for($x=0;$x<$maxwert;$x+=$size){
      $r = rand(0,255);
      $g = rand(0,255);
      $b = rand(0,255);
      $color = imagecolorallocate($img, $r, $g, $b);
     
      imagefilledrectangle ($img, $x, $y, $x+$size ,$y+$size, $color);
    
     }
   }                 
// Save the image
imagepng($img, 'imagefilledrectangle.png');
imagedestroy($img);
?>

My output:

Answer 1

Create two colours outside the loop and then cycle one then the other

<?php
$maxwert = 300;
$size = 20;
$img = imagecreatetruecolor($maxwert, $maxwert);
$colors = [
    imagecolorallocate($img, rand(0,255), rand(0,255) rand(0,255)),
    imagecolorallocate($img, rand(0,255), rand(0,255) rand(0,255))
];


for($y=0;$y<$maxwert;$y += $size){
    for($x=0;$x<$maxwert;$x+=$size){
        imagefilledrectangle ($img, $x, $y, $x+$size ,$y+$size, $colors[(($x/$size)%2+($y/$size))%2]);    
    }
}                
// Save the image
imagepng($img, 'imagefilledrectangle.png');
imagedestroy($img);
?>